Suchitran there is no fixed rank cut-off for any branch in any of the IITs. It all depends on the demands of the students and that's why closing and opening rank is never constant, it varies every year. Last year the opening rank of Aerospace Engineering in IIT Bombay was 749 and the closing rank was 2036 whereas in 2016 the opening rank was 838 and the closing rank was 1841. So, in order to make sure that you get aerospace engineering branch in IIT Bombay you should get a rank atleast in top 1600.

Feel free to ask if you have any more queries. All the best!!